The WiMAX standards (acronym for “Worldwide Interoperability for Microwave Access”), which is a family of standards defining high-speed RF connections intended mainly for point-multipoint architectures, may be cited in this regard. The WiMAX standards comprise in particular the standards of the 802.16 family.
The standards of the 802.11 family, also known by the name “WiFi standards”, may also be cited. The IEEE 802.11 standard is an international standard describing the characteristics of a wireless local area network (WLAN) capable of communicating at high speed over a radius of several tens of meters indoors.
The production of a receiver capable of being compatible with several transmission standards generally requires the ability to process multiple sampling frequencies of the digital signal. Because of the frequency synthesis constraints, the sampling frequency of the signal at the level of the analog/digital conversion stage is not necessarily flexible.
Moreover, analog/digital converters of low power, that is to say of low current consumption, make increasing use of oversampling to reduce the number of quantization levels while maintaining a good signal/quantization noise ratio. This type of analog/digital converter therefore generally requires that the quantization noise be filtered outside of the useful band of the signal and that the sampling frequency be reduced to a lower value than that at which the digital signal is processed. Therefore, conventional sampling frequency conversion devices generally require a certain number of multiplications at high frequency, which leads to considerable complexity of the device and consequently to significant power consumption.
Moreover, it is desirable to be able to perform a sampling frequency conversion in a non-integer conversion ratio, so as to be capable of providing a sampling frequency that is compatible both with the desired transmission standard and with the sampling frequency of the analog/digital conversion stage.
Several approaches exist for performing a sampling frequency conversion with a non-integer conversion ratio.
A first approach includes converting the digital signal into an analog signal, using a digital/analog converter, then filtering the analog signal, and thereafter reconverting it into a digital signal with the desired sampling frequency. Such an approach is hardly satisfactory since it leads to significant conversion noise and there is hardly a satisfactory method for generating the clock signal of the analog/digital converter allowing the reconversion of the analog signal to the desired sampling frequency.
Another approach includes oversampling the digital signal by a factor S, in filtering at the same time the signal aliasing components and the quantization noise, and in thereafter performing a decimation (sampling frequency down conversion), by a factor M. A sampling frequency conversion in a rational ratio S/M is then obtained. However, such an approach is not truly flexible. Indeed, only a small number of conversion ratios can be foreseen, since each conversion ratio may require the production of a filter. Furthermore, generally only rational frequency conversion ratios (ratios whose numerator and denominator are integers) can be used. Furthermore, with operating frequencies currently already being very high, it is particularly difficult to carry out an oversampling without increasing the power consumption and the overall complexity of the device.
Another approach resides in an interpolation, or in a calculation of new samples (interpolated samples) on the basis of the input samples by using an interpolation filter. However, such approaches are currently complicated and use significant memory resources, as well as high-speed buses, for storing all the coefficients of the filter as a function of the temporal deviation between an interpolated sample and the input samples, which serve to calculate this interpolated sample.
Moreover, in certain applications, stages of the delta-sigma type are advantageously used for the analog/digital conversion stages for generating the digital signal whose sampling frequency is to be converted. Such analog/digital converters of delta-sigma type are particularly beneficial since they make it possible to code the digital signal on a low number of bits while rejecting the quantization noise outside of the useful band of the signal. The current approaches for sampling frequency down conversion (decimation) are not suited to oversampled signals originating from converters of the delta-sigma type since the current approaches generally perform only a filtering of the signal aliasing components, but not of the quantization noise.